Speaker
Description
Beta-decay events from radioisotopes produced by cosmic-ray muon spallation are a major background for underground neutrino experiments, especially in regions targeting a few to a few tens of MeV, such as Super-Kamiokande (SK).
The SK detector has been upgraded by loading gadolinium (Gd) into its pure-water target, marking the start of the SK-Gd project. A major physics motivation for the SK-Gd is the first detection of the Diffuse Supernova Neutrino Background (DSNB) in the energy region where spallation events dominate.
Therefore, an accurate understanding of the spallation background produced by the interaction between muons and oxygen nuclei in water is essential for searching for DSNB in SK-Gd.
SK-Gd searches for inverse beta decay (IBD) events from electron antineutrinos in the DSNB by detecting a positron signal accompanied by a delayed neutron signal.
Neutron tagging by Gd suppresses sizable background events without a neutron signal, such as most radioactive-isotope decays.
However, some isotopes, such as lithium-9, undergo beta decays accompanied by neutron emission; thus, these events inevitably consistude an IBD-like background.
Accurate estimation of their production rates and reduction efficiencies, as well as the development of efficient reduction methods for these isotope decays, are important for the DSNB search.
In this poster, we evaluate the amount of spallation background and its reduction efficiency in the DSNB search, employing a detector Monte Carlo simulation for the first time in SK-Gd.